This invention relates to the art of preparing stable aqueous suspensions of reactive pigments obtained by the acid treatment of montmorillonite clay. Such suspensions are adapted for use in the manufacture of coated paper which is used as receiving members of so-called "carbonless coating paper" such as described in U.S. Pat. No. 3,963,852 to Baxter.
Acid-treated reactive montmorillonite pigments are acidic pigments used commercially in the production of carbonless copying paper. The Baxter patent teaches that acid treatment improves the rheological properties of aqueous suspensions of montmorillonite-type clay whereby the treated clay is more amenable to application to paper by present-day paper coating technology. Acid treatment also confers desirable imaging properties when certain dye precursors are used in the manufacture of the record material. Nevertheless such pigments present problems not encountered in the handling of the more familiar coating clay (hydrous kaolin clay). Hydrous kaolins of suitable grade can be made down into fluid high solids suspensions, typically containing about 70% solids. Condensed phosphate salts such as tetrasodium pyrophosphates or Calgon.RTM. are employed as dispersants. These suspensions are mixed with suspensions of adhesives such as latex or starch to form concentrated (for example, 60% solids) suspensions, known as "coating colors." The colors are dried and coated on paper. The rheology of kaolin colors is such that high speed coating machinery, for example blade coaters, can be employed. It is not possible using presently known methods to produce flowable 70% solids dispersed slurries of acid-treated montmorillonite pigments. In fact, some dispersed suspensions containing more than 40% acid-treated clay solids and prepared with the conventional dispersants tend to set up into heavy gels which cannot be used to produce coating colors. Somewhat higher solids suspensions can be produced by using a mixture of a major amount of acid-treated montmorillonite with calcined kaolin clay but these suspensions tend to be dilatant. In other words, the suspensions thicken as shear rate is increased. In some cases a fluid suspension can become so viscous that the blades of the mixing equipment are broken. Dilatancy results in difficulties in pumping as well as mixing and application to paper.
Conventional aqueous suspensions of acid-treated montmorillonite carbonless pigments have acidic pH values. When conventional (phosphate) dispersants are used to prepare high solids suspensions, the pH of the suspensions is slightly acidic. The pH must be increased to about 6.5 or above before makedown into a coating color. Ammonia is conventionally used and this frequently results in severe "shock" -- the transformation of a fluid system into a useless solid or semisolid mass.
Alkanolamines such as AMP have been suggested for use as possible dispersants in the production of high solids (60 to 70%) slurries of calcined kaolin clay. Reference is made to U.S. Pat. No. 4,017,324 to Eggers. The calcined clay must be used with a lesser amount of conventional hydrated kaolin clay. According to the teachings of the patent, the alkanolamine or other dispersant is employed in amount of 0.2 to 2% of the total slurry weight.